There are basically seven different types of roof rafter designs used to create a roof: common, hip, hip jack, valley, valley jack, cripple jack and flying hip. While the most common gable roof can be constructed using only one type of roof rafter, the most complex roof designs can use a combination of all seven roof rafter types. While the rafter can be assembled on site from raw materials, the most common method of building a roof uses a pre-made rafter delivered to the job site by a lumber distributor who will also place the rafters on the roof with a boom truck.
The common rafter is used to create the basic gable roof. This type of rafter runs from an exterior wall and continues up to the ridge board or the peak of the roof. The common roof rafter is used to determine the height of the roof and to place the ridge board. Once the ridge board has been set, the roof is ready for the next type of rafter.
The hip rafter runs on a 45-degree angle to the common rafter. The hip rafter is placed on the corner of the wall or foundation and runs up to the ridge board. This type of rafter is used in finishing off the end of the roof where it meets the end of the building. The hip jack rafter sits on the building's wall and runs up to the hip jack. The hip jack runs in line with the common rafters and lines up with the common rafters on the hip rafter.
Valley rafters are used anywhere there is an inside corner on the roof. Placed at 45-degree angles to the common rafters, the valley roof rafter runs from the ridge board to the outside wall of the building. Valley jack rafters run from the valley rafter up to the ridge board and are in line with the common rafters. The cripple jack rafter is used to span the distance from a valley rafter to a hip rafter when they are located very close to each other.
The flying hip rafter is used on multiple level roofs when there are multiple roof ridge boards at different heights. The flying hip rafter runs from the area where a valley rafter meets up with a ridge board and runs to the end of a higher ridge board. The flying hip roof rafter is also known as a mystery hip rafter. When building a roof it is likely that a combination of several types of rafters may be used in order to complete the finished roof.
How To Figure Roof Rafter Length
Since a roof structure is basically a triangle in form, figuring the length of the rafters of a house will require the use of a familiar mathematical equation, the Pythagorean theorem. To find rafter length, it is necessary to know the rise and run of the roof.
The rise is simply how tall the roof will be at its highest point, measured from its base. The base in this case will be the floor of what will become the attic space.
The run of the roof is half the span or the width of the building. Span can be easily measured or it can be found on the blueprints of a structure.
Once these numbers are found, they can be plugged into a formula to find the length of rafter needed: Rafter length = √ [ (rise)2 + (run)2 ].
Alternatively, there are many online construction calculators that will simplify the process and will not require the builder to take the time to compute mathematical formulas.
How To Repair a Broken Roof Rafter
Replacing a roof rafter is costly and can usually be avoided unless the damage is extensive. In the case of wood rot or invasive pests, the damage may extend to large areas of the rafter. In these instances, it will likely be necessary to replace an entire rafter. By the time rot is obvious in a roof rafter, chances are that the roof structure has been affected as well. Owners should contact a professional contractor to assess what damage has been done, evaluate the need for other professionals such as a roofing contractor or pest control, and replace the damaged rafter.
Common Causes of Rafter Damage
It is more common to discover that a rafter has been damaged and will need repair. Many factors can lead to the damage of roof rafters.
Consistent weight bearing down on a section of the roof can cause damage over time. This could be the case in cold climates when heavy snow builds up on the roof and temperatures prevent melting and runoff. This may also occur when a tree grows in such a way as to rest the weight of a limb on the roof.
This type of damage is often seen in the aftermath of a severe storm. When high winds cause a tree to fall against the roof or other airborne objects to impact it, that sudden strike can cause damage to a rafter even if the external damage seems minimal.
Flaws in the Wood
Even hardwoods can contain inherent flaws that will cause areas of weakness in the beam. If the wood used to build a rafter has existing weak spots, over time the pressure on the wood may cause this flaw to crack or split the rafter.
It is possible that other work on the roof structure can cause damage to the rafters. For instance, a roofer might cause too great of an impact on one rafter causing it to crack, or someone working on ductwork could accidentally cut into a beam creating a weak spot that will worsen in the future.
The most common rafter repair involves the placement of a patch, sometimes referred to as scabbing:
- If the damage has resulted in the rafter sagging, you will need to use a construction jack to push the rafter back into its desired position before repairing it.
- Cut two braces from a length of hardwood. Braces should match the width of the original rafter and extend at least a foot past the damage on both ends. Longer braces will provide more strength and some professionals suggest extending as much as four feet from the site of the damage.
- Effectively sandwich the damaged rafter by adhering the braces on both sides of the rafter with construction glue and then screwing all three pieces together. Screws should be placed every 6 inches along the length of the repair.
How To Reinforce a Roof Rafter
Especially for those living in an area that receives heavy snow or is known to experience severe storms, it may be prudent to reinforce roof rafters before damage has a chance to occur. This can be done in a variety of ways:
- Double the width of the rafter by using two beams rather than one
- Install extra rafters between the originals
- Build a supporting wall underneath the ceiling joists
- Install larger ceiling joists
- Add verticle support posts between the rafters and the ceiling joists with a horizontal support beam